1. Field of the Art
This invention relates to a lens assembly which is particularly suitable for use as an optical objective lens system on an endoscopic insertion instrument.
2. Prior Art
For insertion into a body cavity, endoscopes generally have an insertion instrument which is extended out on the front side of a manipulating head assembly. An illumination means and an endoscopic observation means are incorporated into a distal end portion of the insertion instrument for endoscopic observation and examination of intracavitary sites of interest. The illumination means includes a light guide which is constituted by a bundle of fiber optics to transmit illumination light to a distal end of the insertion instrument from a light source to which an endoscope is disconnectibly connected. A light emitting end of the light guide is disposed in an illumination window which is provided on a rigid tip end section of the insertion instrument. An illumination lens is fitted in the illumination window to disperse illumination light from the light guide in a forward direction. The endoscopic observation means is provided for picking up information of an intracavitary site which is under illumination by the illumination window, and for this purpose includes an optical objective lens system. Optical images of an intracavitary site obtained through the optical objective lens system are either optically or electronically transferred to a viewing device. That is, in the case of an optical endoscope, optical images of an intracavitary site are transferred through an image guide to an eyepiece which is connected to the manipulating head assembly to permit an operator to view the optical images therethrough. In the case of an electronic endoscope, images of an intracavitary site are converted into electric signals by the use of an image sensor device and transferred to a video processor and thereby processed into video signals for display on a viewing screen.
No matter whether an endoscope is an optical type or an electronic type, its optical objective lens system of the endoscopic observation means is composed of a plural number of lens elements. A light incident end or face of a light guide is located at the focus of the optical objective lens system in the case of the optical endoscope, and an image sensor device is located at the focus of the optical objective lens system in the case of the electronic endoscope. The plural number of lens elements of the objective lens system are fitted in a lens tube or tubes of a lens assembly block to be fitted in the observation window. In aligning optical axes of the objective lens system and the light receiving face of the image guide or the image sensor device with each other, it is the general practice to fixe the light receiving face in a retainer tube to be fitted and assembled with the lens tube.
In this connection, a first lens element of an objective lens system, which is located in a foremost position within a lens tube, that is, in a foremost position on the side of a subject, is usually constituted by a planoconcave lens element, with a concave side facing inward of the lens tube. Since the first lens is fitted in an exposed state on a rigid tip end section of an endoscopic insertion instrument, its exposed outer surface is often contaminated by deposition of body fluids or other contaminants which degrade observing picture images to a considerable degree. Therefore, usually a lens surface cleaner is provided on the endoscopic insertion instrument thereby to wash away contaminants from the lens surface. The lens cleaner includes a fluid nozzle which is arranged to spurt a wash fluid toward an exposed outer surface of a first lens element. A wash liquid or cleaning liquid, which is normally water, is spurted toward the outer surface of the first lens, and then compressed air is blasted on the lens surface to remove residues of the wash liquid.
In a body cavity, the first lens element which is located at the distal end of an endoscopic insertion instrument is heated almost to the same level as the body temperature of the patient, and, in the case of an electronic endoscope, heated to a higher level due to existence of a heat generating device such as solid-state image sensor device or the like. On the other hand, a lens cleaning liquid is supplied without heating, that is, a cleaning liquid of approximately ambient temperature is supplied through the lens cleaner. Therefore, the first lens element is cooled off quickly upon sprinkling a cleaning liquid on its outer surface. When the first lens element is cooled off, the inner side of the lens can be fogged by condensation of water vapor in case moisture exists within the lens tube. Besides, since the concave surface on the inner side of the first lens element is of a large curvature, the temperature drop and condensation of moisture occur conspicuously in a center portion of the lens which is far thinner than peripheral portions. Fogging even in a slight degree of a center portion of the first lens element causes considerable degradations to picture images and makes the endoscopic observation difficult because light fluxes necessary for forming images are concentrated in the center portion. A distal end portion of an endoscopic insertion instrument can be cooled off quickly due to other reasons, resulting in fogging of and condensation of moisture on the first lens element of the objective lens system.
Therefore, an optical objective lens system to be incorporated into an endoscopic insertion instrument needs to have an anti-fogging function thereby to prevent fogging or condensation of moisture on lens elements in a lens tube, particularly on a first or foremost lens element of the objective lens system. For this purpose, dry air or nitrogen gas or other moisture-free gas is sealed in the internal space of a lens tube. In addition, in order to maintain a dry state and to prevent intrusion of moisture from outside, it is necessary to seal up fitting portions of a first lens. In forming a seal of this sort, it has been known in the art to fill silicon or other adhesive in a gap space between inner periphery of the lens tube and circumferential surface of the first lens element, for example, as shown in Laid-Open Japanese Patent Application H9-234183. However, considering the severe conditions in which endoscopes are used, that is, considering repeated washing, disinfection and heated sterilization of endoscopes in use, it appears almost impossible for an adhesive or seal material to prevent moisture from creeping into a lens tube of an objective lens system over a long period of time.
In this regard, proposed in Laid-Open Japanese Patent Application 2000-193892 is An air-tight structure which is formed by soldering together a lens tube and a first lens element of an optical objective lens system. In this case, as compared with an adhesive or seal material, the use of metal solder makes it possible to form an extremely sturdy air-tight structure. In addition, because of freedom from drops in air-tightness over a long period of time, it can securely prevent moisture from creeping into the lens tube from outside through the fitting mount portion of the first lens. In this connection, since the lens tube is formed of a metal, solder can be easily applied thereto. However, on the part of the lens element of glass, solder can be easily detached from the lens element if applied directly on a glass surface. Therefore, in order to prevent this, a soldering lens surfaces need to be metalized before applying solder thereto.
The air-tight structure by soldering is advantageous because it can maintain air-tightness stably over a long period of time. However, due to a difference in thermal expansion rate between metal and glass, detachment of solder or damages to lens elements may occur, for example, when the endoscopic insertion instrument is heated in an autoclave for sterilization. For this reason, it is undesirable to solder a circumferential surface of a lens element to an inner peripheral surface of a lens tube.
In the case of the above-mentioned air-tight structure by prior art, an annular lens mount wall is projected on and around the inner periphery of a lens tube, and a lens element is set in position on the lens tube by abutting same against an lens mount surface on one side of the annular wall and then soldered to the latter. In this case, a clearance corresponding to a difference in thermal expansion rate is provided between circumferential surface of the lens element and inner periphery of the lens tube. This however makes it difficult to align the optical axis of the first lens element with that of the lens tube and other lens elements of the objective lens system. Besides, air-tightness seal between the first lens and the lens tube is effective for blocking intrusion of air from front side but not effective for blocking air from back side of the first lens element or from inside of an insertion instrument. Therefore, if moistened air exists within the insertion instrument, it can result in condensation of water vapor on the back side of the first lens element. Normally, an endoscopic insertion instrument as a whole is not hermetically sealed in a moisture-proof state, so that it is difficult to maintain a completely moisture-free dry atmosphere within the insertion instrument. As a consequence, it is usually the case that the inside of a lens tube can be kept in a completely moisture-free state only a limited period of time.
It is an object of the present invention to provide a lens assembly suitable for an endoscopic optical objective lens system, having spaces between adjacent lens elements of the system hermetically sealed in a reliable manner.
It is another object of the present invention to provide a lens assembly suitable for an endoscopic objective lens system, having spaces between adjacent lens elements hermetically sealed in such a way as to prevent intrusion of moisture over a long period of time.
It is still another object of the present invention to provide a lens assembly suitable for an endoscopic objective lens system, which can prevent condensation of water vapor on an inner surface of a first or foremost lens element of the system.
It is a further object of the present invention to provide a lens assembly suitable for an endoscopic objective lens system, having an air space between first and second lens elements of the system hermetically sealed in a reliable manner.
According to the present invention, in order to achieve the above-stated objectives, there is provided a lens assembly for an endoscopic optical lens system to be incorporated into a distal end portion of an endoscopic insertion instrument, the lens assembly comprising: metal film layers formed on and around confronting outer peripheral surface portions, outside effective lens areas, of at least two adjacently located lens elements of the optical lens system; and solder applied to and between the metal film layers to bond said adjacently located lens elements fixedly to each other in a hermetically sealed state.
From the standpoint of preventing fogging, it is important to prevent condensation of water vapor on an inner surface of a first lens element which is located in a foremost position, on the side of a subject, among a plural number of lens elements which constitute an endoscopic optical lens system. Therefore, above all, an air space between a first lens in a foremost position and a second lens in a second foremost position of a lens system is hermetically sealed by the use of solder which fixedly bonds the first and second lenses together. In order to enhance the bonding strength by solder, metal film layers are formed on confronting outer peripheral surfaces, outside effective lens areas, of the first and second lenses. The metal film layers can be formed, for example, by an evaporation process. However, in order to form the metal film layers by a simpler method; it is desirable to employ a metal plating process. More specifically, the metal film layers can be formed by electroless nickel plating, followed by electroless gold plating.
In soldering lens elements together, a flux can be filled into a joint portion for the purpose of distributing solder more positively. If a flux is charged directly between metal film layers of two adjoining lens elements, it may get as far as an air space between the two lens elements. In order to avoid this, it is preferred to form a seal around the joint portion between two lens elements by applying an adhesive at the time of assembling and adjusting the optical axes of the two lens elements into alignment with each other. The use of an adhesive in this way contributes to fix the two lens elements temporarily to each other prior to soldering, and to form a seal or shield portion on the inner side of a joint portion to block intrusion of solder and flux.
In this connection, in assembling the first and second lenses of an optical lens system, two lens elements are either directly bonded to each other or indirectly bonded to each other through a spacer member. In the former case, a V-groove is formed between confronting outer peripheral surfaces outside of effective lens areas of the respective lenses to be joined, and solder is applied to the V-groove to fix the two to each other. In the latter case, that is, in a case where an annular spacer member is interposed between first and second leases, a V-groove is formed on each side of the spacer member, that is, between the first lens and the spacer member and between the second lens and the spacer member, and solder is applied to each V-groove to bond the first and second lenses fixedly to each other through the spacer member. In the latter case, in order to distribute solder to each side of the spacer member, it is desirable to provide notched portions at outer peripheral edges of the spacer member or to perforate a number of holes in the spacer member. Preferably, an adhesive is applied between the first and second lenses before soldering to fix the two lenses temporarily in an aligned state and to form a seal or shield in and around bottom portions of the V-groove block solder and flux which might otherwise intrude into an air space between the first and second lenses. In a case where an endoscopic objective lens system is constituted by three or more lens elements, similarly a third or other additional lens elements can be soldered to the first and second lenses which are assembled by the use of solder in the manner as described above. Furthermore, the lens assembly according to the present invention can be similarly applied to an illumination lens system which is composed of a plural number of optical lens elements.
The above and other objects, features and advantages of the present invention will become apparent from the following particular description, taken in conjunction with the accompanying drawings which show by way of example preferred embodiments of the invention. Needless to say, the present invention should not be construed as being limited particular forms shown in the drawings.